An on-site utility detection and mark out survey is one in which the locations of potential utilities or other underground features are marked in real time using chalk, crayon or temporary paint. In this case it is necessary for an experienced surveyor to perform the survey using GPR and EML who is competent in the use of EML and can interpret the GPR data displayed on the screen in real time.
During a utility survey with onsite mark out, the advantage that it the results are determined in real time and there is a low false alarm rate. On-site information is available to help quantify the results (ie. you can see a manhole and then track the pipe which leads from it – so you will know that that the pipe is truly present and what type of utility it represents). For some types of survey with a requirement for the utilities to be marked on the ground in paint, on-site detection is by far the most reasonable option.
The main disadvantage of an on-site detection survey is that whilst using the GPR only scan can be viewed at a time (compared to several in post processing), so there is less GPR information available to help the decision-making process and it will be more difficult to detect less obvious utilities, which might have been easier to detect during office based data processing and interpretation with the ability to view multiple GPR scans at the same time. It is also much more difficult or impossible to subject the survey results to a meaningful quality control because the results are determined in real time.
If a drawing is required then these locations will be collected using a GPS or Total Station and added to the drawing in the office, but in many cases it is the paint marks themselves which are the final deliverable.
Post processing is the (incorrect) term generally accepted in the UK to refer to a GPR survey in which the raw GPR data is collected and saved on site, and then processed, reviewed and interpreted later in the office using a separate computer and software package. Depending on the utility density, a post processed GPR survey may be more time efficient than an on-site mark out survey because the operator is just collecting the data rather than also taking the time to review and make decisions based on it.
During a post processed survey, a GPR operator will attend site and collect a series of parallel lines of GPR data, usually in a grid. In the UK, most GPR surveys are specified with a grid of predetermined spacing, with respect to PAS128, although this is not always the case. Generally speaking, the closer the spacing between the lines of GPR data, the easier it will be to identify and follow particular features below the ground. In addition, closely spaced lines of GPR data can be combined to produce images which can aid in data interpretation.
As the spacing between the lines of data increases, the ability to produce useful images is quickly lost and there is less value in post processing, however in many cases good imaging is not necessary to produce the required results. The GPR operator should decide how to undertake the survey based on his experience so that the correct results are achieved and advise the customer accordingly.
The advantages of a post processed GPR survey are that less time is spent using GPR on site and that much more information is available for the decision-making process during data interpretation; multiple GPR lines can be visualised at the same time and represented in different ways to emphasise different features. The results from a post-processed GPR survey will usually be more detailed and they can be subjected to a quality control before delivery to the customer. As a result, a post processed GPR survey is usually considered to be higher quality than an on-site mark out survey.
The EML portion of the survey will be completed on-site as normal, with the results recorded and then put onto a drawing. These results will then be compared and combined with the GPR results during the post processing phase.
Results from data processing will be put onto a drawing for delivery to the customer.
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With GPR, you can detect a wide range of objects below ground level, including both metallic and non-metallic objects such as plastic pipework. GPR will also identify and map any voids below the surface, such as air pockets or mine shafts, as well as any other irregularities including concrete and previously excavated or back-filled areas.
GPR equipment emits an electromagnetic pulse into the ground and records the reflected signals from subsurface structures and voids. It is entirely non-destructive and will not break the ground’s surface or affect any objects below. What’s more, it doesn’t emit any harmful levels of radiation, nor are there any other by-products created throughout the process. This means it’s entirely safe to use by its operators, and on sites of any type, including those open to the public.
While GPR is one of the most effective methods of non-destructive testing available, it can never be 100% accurate. One factor that can adversely affect the accuracy levels include the type of soil being surveyed. Clay soils and soils that contain high levels of salt or minerals can obstruct the GPR reading. Another factor is the experience of the equipment’s operator: interpreting the data collected can be complex, which is why it’s beneficial to commission surveys from an expert firm.
The equipment itself is not difficult to use, but the interpretation of the data recorded tends to be complicated. The results of a GPR survey aren’t automatically translated into an easy-to-understand picture of what lies below the surface; instead, it’s a series of lines and waves and it can take both training and years of practice to master the art of correctly reading the output. Often, it is the experience of the equipment’s operator that plays the most significant role in the accuracy of the results GPR can achieve.